Various industrial processes generate waste gases containing gaseous pollutants such as organic solvents. From the standpoint of the reuse of resources or the prevention of air pollution, therefore, a waste gas containing such noxious gaseous pollutants must be freed of the noxious pollutants before it is released into the atmosphere.
Various methods for effecting purification of gases containing gaseous pollutants by adsorption have been devised. As occasion requires, these prior methods involve recovery of the removed noxious pollutants. Of these prior methods, one popular method makes use of a fluidized-bed type adsorption system wherein a gas to be treated and adsorbent particles such as activated carbon, activated alumina or silica are brought into mutual contact to form a fluidized bed of the adsorbent particles. In the continuous adsorption of the gas by this fluidzied-bed method, it is common practice to arrange fluidized beds in a plurality of stages within a tower as illustrated in FIG. 1 of the accompanying drawing, for example. In FIG. 1, 1 denotes a reaction tower. A gas containing noxious gaseous pollutants to be removed is introduced into the tower 1 through a nozzle 2 in the adsorption section A. On entering the tower interior, the gas ascends vertically and comes into contact with adsorbent particles held inside the adsorption section A, causing the adsorbent particles to form fluidized beds on the superposed trays 3, 3', 3" . . . The adsorbent particles forming the fluidized beds adsorb the gaseous pollutants from the gas. The gas which has thus been freed of the noxious pollutants is released into the atmosphere via a discharge outlet 4 at the top of the tower. The adsorbent particles on the superposed trays 3, 3', 3" . . . , fall through the downcommers 5, 5', 5". . . associated with the trays and descent gradually downwardly by virtue of gravity, while simultaneously adsorbing the gaseous pollutants from the gas. Then, they leave the adsorption section A and accumulate in a funnel or guide 6, forming a gravitationally moving bed. The particulate carbon eventually reaches regeneration section B which is located at the bottom of the reaction tower 1. On entering the regeneration section B, the adsorbent particles are heated by a heater 7, with the result that the particles are regenerated as they are forced by the heating to release the adsorbed pollutants. Subsequently, the regenerated adsorbent particles reaching the bottom 8 of the tower are transferred via a lifting pipe 9 to the top of the tower for recyclic service. In the meantime, the pollutants which have been desorbed from the adsorbent particles are forced out of the system via a nozzle 10 by means of a carrier gas being introduced via a nozzle 11 disposed at the lower portion of the regeneration section B. The discharged pollutants are transferred to a desorbate treating section C composed of a condenser, decanter and the like.
In the adsorption treatment of the gas by the fluidized-bed method described above, successful stabilization of the fluidized beds thus formed constitutes an essential requirement for enabling the removal of the noxious gaseous pollutants from the gas to be effected continuously at a high removal efficiency over long periods of service. The stability of such fluidized beds depends on the shape of adsorbent particles used, the strength, wear resistance and other physical properties of the particles and the flow volume, flow velocity and viscosity of the gas used for fluidizing the adsorbent particales, and so on. It also depends on the extent of change in the weight of the adsorbent particles being recycled. When the adsorption treatment of gas by the convention fluidized-bed type technique is reviewed from this point of view, it is noted that the so-called coconut-shell activated carbon obtained from coconut husks is popularly used as the adsorbent particles. The activated carbon of this type is made up of particles of various, complicated shapes and therefore makes their transport substantially difficult. Moreover, the adsorbent particles have poor physical properties and, for this reason, are readily pulverized as by crushing and attrition. Recyclic use of such activated carbon particles of irregular shapes, therefore, involves numerous difficulties. In the adsorption treatment of gas by the fluidized-bed method, the adsorbent particles of such shapes induce undesirable phenomena such as boiling, channeling and slugging, when fluidized by the upward flow of the gas under treatment. They also cause similar phenomena while they are moving downwardly via the downcommers (corresponding to the items denoted by 5, 5', 5", . . . in FIG. 1) by gravity, with the result that smooth flow of the particles inside the downcommers is impeded. This impeded flow consequently brings about a quantitative change in the weight of the adsorbent particles being transferred for recyclic service. With a view of precluding these disadvantageous phenomena, the conventional approach has been directed to improvement of the structure of downcommers of the particles. For example, U.S. Pat. No. 2,674,338 discloses bottom plates supported on springs on the bottoms of the downcommers. These attempts at improvement of the structure of the downcommers, however, effectively complicate the system itself and have the disadvantage that activated carbon particles will gradually change in shape with the lapse of time. Thus, all these attempts have failed to attain the desired stabilization of the quantitative transport of adsorbent particles. Because the adsorbent particles in use are highly susceptible to pulverization and also because stabilization of the transport of these absorbent particles is difficult to accomplish, the conventional techniques do not easily fulfill the objective of stabilizing the fluidized beds of the absorbent particles.